This invention relates to solar heating systems and more particularly relates to efficient absorption and retention of solar energy. It especially relates to coatings, to be applied to aluminum surfaces, having high selectivity for solar energy absorption.
The concept of using solar energy as a source of heat is probably as old as mankind itself and the usual procedure was to heat a black surface by the rays of the sun in order to extract heat therefrom.
Solar radiation reaching the surface of the earth is mainly concentrated in the visible spectrum and does not exceed a wave length of about 2 microns. On the other hand, the long-wave thermal radiation spectrum is greater than 3 microns so that there is virtually no overlapping of the shortwave solar spectrum and the longwave thermal spectrum. Because of this rather recent realization, the concept of utilizing selective surfaces for solar collectors has received added attention. Selective surfaces are those whose absorptance and emittance vary with the wave length of the incident radiant energy. Thus, in order to be a good solar collector, a surface exposed to solar radiation must be as little light-reflecting and as little transparent to light as possible; that-is, as dark as possible, preferably black. However, it can generally be stated that the darker a body, the more heat it radiates with increasing temperature, the heat radiation having wave lengths above about 2 or 3 microns. Therefore, the better the receiver is adapted to absorb solar radiation, the greater the energy heat losses by heat radiation with the result that it had not been possible until recently to obtain utilizable energy at high temperature from solar heaters, except by the use of optical systems of high concentration power.
Black paint has long been the most utilized of the non-selective coatings, that is, it is a coating whose absorptance and emittance remains constant over the entire spectrum of thermal and solar waves. Black paint performs reasonably well up to a collector temperature of about 150.degree. C., but above this temperature, its efficiency declines due to thermal losses from re-radiation. Paint also has the serious disadvantage of being an organic material subject to deterioration under normal weather and solar conditions. The main advantage of black paint collectors is their ease of manufacture and low cost.
Recently, there has been made available to the art solar coatings which are quite selective, i.e., materials that have a high absorption for solar radiation and at the same time exhibit very low emissive losses for thermal radiation.
One such coating involves aluminum or an alloy thereof which has been anodized and thereafter blackened by reaction therewith with a copper salt solution. A coating of this type is disclosed in U.S. Pat. No. 2,917,817. This coating is stated to be selective and particularly adapted to be used as a solar collector.
However, there are other physical characteristics of a solar coating which must be taken into consideration aside from its absorptance and emittance characteristics. Thus, a solar coating should be stable. It should not degrade with time and it should be able to be easily reproducible so that it is adaptable for commercial manufacture.
It is precisely in this area that the heretofore employed prior art coatings have suffered serious drawbacks. Thus, for example, it is known in the art that the .alpha. and .epsilon. characteristics of certain coatings, including aluminum oxide coatings, are directly related to the thickness of the coating. By way of considerable oversimplification, it can be stated that a thick oxide coating is undesirable from the point of view of its .alpha. and .epsilon. characteristics. However, when using a thin oxide coating produced by anodizing aluminum, severe mechanical properties have arisen, particularly when the necessary blackening solution was employed. A thin anodized layer of aluminum oxide which has been blackened is subject to irregularities in the surface characteristics thereof, thereby detracting from its potential for use as a solar collector.
Relatively hard, porous, adherent, and adsorbent coatings of such metals as copper, silver, and gold have been formed on aluminum by first anodically oxidizing an aluminum article in an electrolyte such as sulfuric acid, chromic acid, or oxalic acid and then depositing completely reduced metal thereon, as disclosed in U.S. Pat. No. 1,988,012. However, lighter colored silver deposits include unreduced or partially reduced metal salts so that the coatings change color when exposed to sunlight.
For present purposes, the term "aluminum" is used with reference to the metal itself and alloys composed predominantly of aluminum, including commercial grades of aluminum with ordinary impurities and normal wrought alloys which may also contain added elements in relatively minor amounts, especially those alloys containing upwards of 90% aluminum by weight.
One measure of the selectivity of a particular surface is the ratio of the shortwave solar absorptance to the longwave thermal emittance. This ratio, however, does not always adequately define the best absorptance-emittance characteristics for a practical solar collector. Thus, for instance, various surfaces of sufficiently low emittance may have high .alpha./.epsilon. ratios and yet exhibit only moderate absorptance. It has been found desirable, therefor, to consider the difference factor (.alpha.-.epsilon.) as an additional indicator.
Among the best state-of-the art solar coatings are black nickel and black chrome, which typically have an .alpha./.epsilon. ratio of about 10:1. They are technically quite suitable, but rather expensive to produce. Other surfaces also exhibiting high .alpha./.epsilon. ratios sometimes do not have adequate properties to qualify as practical solar coatings. Polished zinc, for example, may have an absorptance of 0.5 and emittance of 0.05, giving an .alpha./.epsilon. ratio of 10:1. Similarly, bare aluminum exhibits typical .alpha./.epsilon. values up to 15:1 depending on surface condition. Neither of these surfaces has an acceptably high absorptance even though their .alpha./.epsilon. ratios are very good.
Absorptance values are conveniently measured using a Gardner Modified Hazemeter with a filter giving the maximum intensity at about 5560 A, the maximum visible wavelength. Emittance may be determined at 300.degree. F., using a Gier Dunkle Total Normal Emittance system.
The natural oxide of aluminum formed on its exposed surfaces is not a desirable attribute for solar heat receptor devices because such surfaces, although having a rather high absorptance/emittance ratio (.alpha./.epsilon.), tend to exhibit a low absorptance of only about 0.40, i.e., their high .alpha./.epsilon. ratio comes about only by virtue of an even lower emittance characteristic. Moreover, the difference (.alpha.-.epsilon.) between their absorptance and emittance is typically less than 0.40.
It has been recognized that absorptance can be increased appreciably (at least doubled) by use of black paint on aluminum surfaces. However, the .alpha./.epsilon. ratio obtained is closer to unity, since the emittance of such painted surfaces is about the same as their absorptance.
Other ways have been proposed to solve this problem, in order to be able to exploit other desirable properties of aluminum, especially its good heat conductivity, ease of forming into fabricated articles, and relatively low cost. Perhaps the most successful, although quite costly, is to use plating operations to deposit one or more layers of nickel or other metals of suitable properties.
Thus, for example, nickel on aluminum is effective to achieve a moderate absorptance of about 0.55 and emittance of about 0.15 (or typically .alpha./.epsilon. = 3.7 and .alpha.-.epsilon. = 0.40); polished zinc on aluminum achieves about the same absorptance and lower emittance, hence a higher .alpha./.epsilon. ratio (typically 10:1) but about the same difference (.alpha.-.epsilon.). So-called black nickel and black chrome show higher absorptance (0.87 to 0.88) and low emittance (0.07 to 0.11) for a typical (.alpha./.epsilon.) ratio of 10.0:1 and a typical .alpha.-.epsilon. difference of 0.8. Black anodized aluminum has excellent absorptance but relatively high emittance (.alpha./.epsilon. about 1.2, with .alpha.-.epsilon. of only about 0.20). As has heretofore been mentioned, its absorptance and emittance characteristics can be improved only at the expense of its mechanical properties.
From the foregoing, it may be noted that various approaches have been available for increasing absorptance or decreasing emittance, but those able to do both tend to be prohibitively expensive for routine use.
Based on the .alpha./.epsilon. parameter, a black nickel coating, mill-finished aluminum, and polished zinc all appear to be suitable and much better than some others. Using the .alpha.-.epsilon. parameter, however, the relative merits of various surfaces are more readily apparent.
The basic structural elements of apparatus suitable for purposes of the invention are a sheet or panel of relatively high surface area compared to other dimensions, usually having fins or the like to increase the effective area, and means such as tubular conduits formed integrally or otherwise disposed in heat exchange relationship therewith for transmitting fluid to extract heat. The fluid used is normally water or an aqueous solution of lower freezing point, but it may also be a gas such as air.
For purposes of background information on related aspects of solar heat collection, reference is made to "Low Temperature Engineering Application of Solar Energy," published by American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (especially Chapter IV on Selective Surfaces for Solar Collectors); and, on the subject of chemical oxide coatings for aluminum to "The Surface Treatment and Finishing of Aluminum and its Alloys," by Wernick and Pinner. 1959, published by Robert Draper Ltd. (especially section 5 on Chemical Conversion Coatings, beginning at page 166).